Hexagonal grids used in paintings. Drawing the grid with a compass, the first two points determine the entire grid. Because the grid is inherently three-dimensional, it works as a sort of perspective.
Heraclitus: The hidden harmony is better than the obvious.
The Painter’s Secret Geometry 117:
In the Middle Ages the ‘geometry’ of a work of art, whether picture, bas-relief or page of manuscript, consisted chiefly in the use of the regular polygons as an armature, as an interior framework, figures that were sometimes quite complicated, with five, six, or eight sides, not forgetting the double figures formed by the star pentagons and hexagons.
…In spite of its strictness, geometry as understood in the Middle Ages was no hindrance to the imagination.